Generating light beams or thin light sheets with high Rayleigh lengths is desired for various technical applications in which light beams with a transverse intensity profile as flat as possible over a large length of the light beam are useful. This includes in particular so-called light sheet microscopy, by means of which large-volume specimens can be analyzed three-dimensionally in layers. To this effect, the transparent fluorescent preparation is irradiated from the side with a thin light sheet, whereby an “optical section” is generated. The optical section results from the optical excitation of the fluorescence specifically in the plane of the light sheet. The section is recorded essentially perpendicular to the light sheet plane using a camera. The repetition enables a layered three-dimensional reconstruction of the specimen. A decisive factor for the resolution in the axial direction is the thickness of the light sheet. In the case of conventional Gaussian optics with essentially Gaussian distribution of the transverse intensity profile, the profile of the light sheet quickly becomes broader at increasing distance from the focus. Known alternatives thus use instead of Gaussian beams so-called Bessel beams that are generated with suitable alternative optical means in a manner known per se. Bessel beams have an intensive central intensity maximum. They however have undesirable secondary maxima in the form of coaxial “rings”. These secondary maxima generate a disturbing background, which depending on the excitation behavior of the used fluorescence dye negatively widens the effective thickness of the analyzable optical layer.
Aside from this, the so-called “super-resolution” light microscopy according to the STED (stimulated emission depletion) principle was developed; it is described for example in DE 10 2009 008 646. In STED microscopy, the diffraction limitation of the excitation light is overcome in that a particular excitation light beam is provided which has a central excitation focus and is surrounded concentrically by a further light beam with a circular intensity profile of another wavelength, the so-called deexcitation light beam. Through the combination of a central excitation beam with this deexcitation beam encircling it, in connection with the excitation behavior of the fluorescence dye, the fluorescence emission in the specimen is reduced to a local area, which thereby lies below the diffraction limit of the excitation light itself. The disadvantage of the known optical configurations for generating the concentric STED illumination is the low Rayleigh length of the beams. In known configurations, the effective interaction of the two concentric light beams is thus possible only in one focus point in the beams' optical axis. It is desirable for the Rayleigh length of these concentric light beams to be increased. In particular, this should also make possible the combination of STED microscopy with light sheet microscopy.
Additionally, use of light sheet microscopy on the basis of Gaussian beams from lasers for two-photon excitation at wavelengths in the near-infrared range for irradiating barely transparent specimens thus is hardly practicable due then to the very small focus lengths.